Field of the Invention
The invention relates in general to an acoustic-wave device, and more particularly to an acoustic-wave device with active calibration mechanism.
Description of the Related Art
Referring to FIG. 1, a schematic diagram of an acoustic-wave device 900 is shown. The acoustic-wave device 900 includes a piezoelectric substrate 910, a piezo film layer 920 and an interdigitated capacitor structure 930. Surface acoustic waves are spread on the interdigitated capacitor structure 930. An electric signal is converted into an acoustic signal by the piezo film layer 920, then the acoustic signal is further converted into an electric signal.
Along with the development of the technique of surface acoustic wave, the acoustic-wave device 900 has been used in many fields. For example, the acoustic-wave device 900 can be used as a filter, an oscillator, a transformer and a sensor of a mobile phone. The acoustic-wave device 900 can also be used in the fields of radio and TV, such that the frequency range of radio reception can be very narrow and accurate. Or, the acoustic-wave device 900 can monitor and forecast earthquakes because the surface acoustic wave can be spread on the surface of the earth.
Since the interdigitated capacitor structure 930 and the piezo film layer 920 have different coefficients of thermal expansion, the acoustic-wave device 900 may generate warpage. Referring to FIG. 2A, a schematic diagram of the acoustic-wave device 900 at a low temperature state is shown. When the acoustic-wave device 900 at the low temperature state, the contraction of the interdigitated capacitor structure 930 is greater than that of the piezo film layer 920, so the two edges of the acoustic-wave device 900 will be warped upward. At the low temperature state, the pitch of the interdigitated capacitor structure 930 is reduced, the signals are shifted towards high frequencies.
Referring to FIG. 2B, a schematic diagram of the acoustic-wave device 900 at a high temperature state is shown. When the acoustic-wave device 900 at the high temperature state, the expansion of the interdigitated capacitor structure 930 is greater than that of the piezo film layer 920, so the two edges of the acoustic-wave device 900 will be warped downward. At the high temperature state, the pitch of the interdigitated capacitor structure 930 is enlarged, the signals are shifted towards low frequencies.
Referring to FIG. 3A, an insertion loss curve diagram of the acoustic-wave device 900 under different temperatures is shown. The frequency response curve L11 is an insertion loss curve measured at 20° C., the frequency response curve L12 is an insertion loss curve measured at 50° C., and the frequency response curve L13 is an insertion loss curve measured at 85° C. The three frequency response curves L11, L12 and L13 show that as the temperature increases, the insertion loss gradually drifts towards low frequencies.
Referring to FIG. 3B, a return loss curve diagram of the acoustic-wave device 900 under different temperatures. The frequency response curve L21 is a return loss curve measured at 20° C., the frequency response curve L22 is a return loss curve measured at 50° C., and the frequency response curve L23 is a return loss curve measured at 85° C. The three frequency response curves L21, L22 and L23 show that as the temperature increases, the return loss gradually drifts towards low frequencies.
Apart from the temperature which may cause signal variation to the acoustic-wave device 900, errors in the manufacturing process also cause signal variation to the acoustic-wave device 900. For example, when the pitch of the interdigitated capacitor structure 930 is too small, signals will shift towards high frequencies. On the other hand, when the pitch of the interdigitated capacitor structure 930 is too large, signals will shift towards low frequencies.
As disclosed above, signal variation caused by temperature factor or manufacturing process factor has always been a bottleneck that is hard to overcome. The research personnel in the industries have been dedicated to resolve the bottleneck.